1. Field of the Invention
The present invention is directed to blends of high impact poly(p-methylstyrene) (HIPPMS), and particularly to blends of HIPPMS and a copolymer which impart higher environmental stress crack resistance to fats and fat-containing foods on the resulting polymer.
2. Description of Prior Art
It is well known in the art to produce various shaped articles from foamed and unfoamed polymer sheets by thermoforming methods. Such articles may be used as containers for packaging foods.
It is also known to blend HIPPMS with other polymers. For example, it has previously been disclosed to blend HIPPMS and polystyrene with conjugated polydienes (U.S. Pat. No. 4,352,908), a conjugated diene, such as isoprene or butadiene (U.S. Pat. Nos. 4,260,694 and 4,397,988), or a styrene-conjugated diolefin block copolymer such as styrene-butadiene-styrene (U.S. Pat. No. 4,275,179). The above-mentioned references are incorporated herein by reference. The general purpose of these blends is to create a stronger, more heat resistance product. However, due to the structure of the HIPPMS polymer, containers made from HIPPMS are melted by hot fats and fatty acids and thus cannot be used to package fat-containing foods that are to be heated. It has been discovered that containers made from HIPPMS will become resistant to hot fats when treated by radiation (U.S. Pat. No. 4,205,114). However, until now it has not been known how to form non-radiated HIPPMS containers which may hold fat-containing foods without the above-mentioned adverse effects.
Surprisingly, it has been found that containers made from blends of HIPPMS and an additive polymer having higher than normal environmental stress crack resistance (ESCR) are resistant to the deleterious effects of fat-containing foods and fatty acids.